texts:
For some "average sized" black hole, how long would it take for a distant (spacesuit-wearing!) astronaut to:

be able to see the gravitational lensing with the naked eye
clearly feel the gravity gradient
reach a lethal gravity gradient
cross the event horizon

Ultimately I'm interested in the subjective experience of a human falling into a black hole. Would it be drawn out and unpleasant? Or is it that by the time the gradient would be painful, there'd be barely any time at all before annihilation?


texts: Can concentration of gas in exoplanet atmosphere be found out from wavelength and absorbing radius from spectral data?

texts: I was working on exoplanet spectral data from which I need to infer the concentration of gases. However, the exoplanet spectroscopy data contains only absorption wavelength and absorption radius. Is there any way with which I can derive the concentration of the elements in the exoplanet's atmosphere?


texts:
I want to prove Kepler's Third Law via Vector Analysis of motion. I know of other derivations, but want to know this as I think it will be interesting. I have been unable to prove it. But I know of proofs of 1st and 2nd law via Vector Analysis. Can anyone please tell me how to proceed on it, please? One equation can be
\begin{equation}
\frac{1}{2} \times \vec r × \frac{d \vec r}{dt}  = constant.
\end{equation}
Please, can anyone help me go further?
I tried using $\vec r= acosωt\hat i +bsinωt\hat j$,
So the areal velocity came $\frac{1}{2} abω \hat k$. But then I tried integrating areal velocity over time thinking it would give total area, but the relation of 3rd law didn't come. So can anyone please help?


texts: Would blocking sunlight on Venus help make it habitable?

texts:
Lets say we had the technology to create a mini-Dyson structure, like a shutter style curtain between the Sun and Venus, blocking the sunlight in a controllable manner to cool down it's temperature.
Could it be the first step to reverse the greenhouse effect and make Venus more "terraformable"? Or at least more explorable like Mars? Would there be any unintended consequences?
Would this be useful on other hot extra-solar earth-like planets?


texts: Can we apply Kepler's law of planetary motion to the moons motion around the earth?

texts: I have been trying to understand Kepler's law of planetary motion for a while. So I was thinking, if it would even apply to the moon orbiting around the earth. Does the moon too orbit in an ellipse too? Does its orbit have the same properties as the earth's orbit around the sun?


texts: Calculate Earth's distance travelled in a year

texts:
This is probably more of a math question than astronomy, but I'd like to know the distance the Earth travels in one revolution around the sun (i.e. a year).

According to the data in Wikipedia, I think there may be several ways for me to calculate this but I'm not sure whether they are correct or how to do them:

Calculate by velocity and time: the average velocity is given as $107200\ \mathrm{km/h}$ (a suspiciously round number; imprecise?) and one revolution takes $365.256363004$ days. That gives $\text{distance} = 107\,200\ \mathrm{km/h} \times (365.256363004 \times 24\ \mathrm h) = 939\,731\,570.736691\ \mathrm{km} \approx 939.731\ \mathrm{Gm}$
Calculate by semi-major axis and eccentricity: my math is too weak for that. I searched the web and didn't find a solution for that (calculating the circumference of an ellipse exactly seems to be a problem?)
Any other way using the orbital data given in the Wikipedia article?

In essence, this can be made into a pretty generic question: How do I calculate the distance an object travels in one revolution using the usual orbital parameters?


texts:
Regularly, I see rainbows around the moon. Here is approximately what I see:

Right at the edge of the moon, I see a bright blue ring of light. Further out, I see an inner rainbow. This is not as bright as the ring, but is still clearly visible. Even further out, I see an outer rainbow that is very faint.

I see the blue ring during every full moon. I see the rainbows more frequently in late fall and winter than in spring or summer. But every year, I see these moon rainbows during a full moon.

Since I see these moon rainbows more frequently in late fall and in winter, are these moon rainbows caused by moonlight reflecting off of ice crystals in cirrus clouds high up enough that I can't see the clouds? Also, why do I see a bright blue ring right at the edge of the moon during every full moon?


texts:
Recent news seem to suggest that astronomers are arguing about whether Pluto should become a planet again.

However, I cannot find an official source for this. Is this actually true?


texts: Would radio telescope located anywhere within a 100 light year radius detect our radio emissions?

texts:
If there was a radio telescope, as sophisticated as our best radio telescopes within a 100 light year radius from earth.

AND if its sole purpose was search for signs of artificial/intelligent radio transmissions like our SETI program.

What chance would such radio telescope have of detecting our radio emissions regardless of its three dimensional location relative to earth and how would that chance change the further it is from us?


texts: What kinds of stars would it be possible to observe in M31 with a small amateur telescope?

texts:
I was wondering what kinds of stars would be visible in M31 and other nearby galaxies using a moderate (~8") telescope and a CCD camera. This would be a limiting magnitude of around 17.

I have heard of LBVs (Luminous Blue Variables). However, I would like to know what spectral class of stars would most likely be visible (would it be mainly OBA type stars)?

The context of this question is in the search for extragalactic transiting exoplanets. Current amateur detectors have the capability to detect planets with host star apparent magnitudes of 14. I was wondering if it might be possible to find exoplanets in M31, M33 or even the LMC/SMC with these limits. An advantage to searching for planets in M31 would be the very limited search region--an amateur astronomer would only have to perform photometry on the M31 region over several months or years to find a planet, rather than having to search a much larger region.

Are there any other issues with detecting transiting planets around very luminous stars? Would the percent dip in brightness be far less for a Jupiter sized planet? How would this affect our ability to detect extragalactic transiting exoplanets?


texts: What does it mean when we say that information is potentially recoverable from a black hole

texts:
My understanding is that if the surface of a black hole is holographic, then information is potentially emitted from a black hole, say due to Hawking radiation or some other means.

What can say about this information?  Is it physically possible and/or feasible that this type of emission would embed information about what went into it?  Also, how does this measure up against the idea that the world lines inside the event horizon are completely disconnected from the outside?

I have some knowledge of the information paradox, but I was really hoping someone could  explain it in simple terms or point me to a prior explanation along those lines.


texts: Why weren't Lunar meteorites identified until 1982?

texts:
According to Wikipedia the first meteorite of Lunar origin wasn't identified until 1982. Today 306 Lunar meteorites have been identified.

Why weren't they identified as Lunar already a decade earlier, when
the Apollo samples had been analyzed?
How do Lunar meteorites differ, the Lunar surface consists of
meteoroid material anyway?
Were there suggestions before 1982 that certain meteorites were of
Lunar origin, and did they turn out to be correct?


texts: How is the path of an asteroid calculated?

texts: How do scientists and astronomers recognize the perfect path or orbit (if it's orbiting an object) of an asteroid or a comet?


texts: Isn't it likely that most extra solar planets don't transit their star in line with the Earth?

texts: In which case, for every planet that we detect, there should be significantly more that we can't detect because they don't transit in line with the Earth and, thus, are invisible.  Now, I suppose that we can reasonably assume that most planets in a given solar system orbit in a fairly well defined plane, so if we detect one planet then there are probably others in the same plane.  But, if we have detected no planets in a system, isn't it simply more likely that the planets are orbiting in a plane that doesn't bisect the Earth?  If that is the case, since there is such a tiny degree of variation that would allow a transit to be observed, shouldn't there be a huge number of planets that are not visible because the inclination of their orbital plane is something other than 0 (with respect to the Earth)?


texts: What planet is better than earth to infer solar system configuration?

texts:
The mankind had to work some centuries to infer the real configuration of solar system, starting from greeks, Ptolemeus, until Copernicus, Galilei, Kepler, Newton etc. Is there any planet where we could better/faster determine the configuration?

(For example, having a moon is an advantage. Maybe having two moons(or none) or having a thinner atmosphere or being closer/farther to the sun may helped.)

The area of the question starts from inferring the form of the planet(round) until the discovery and orbits of all eight planets.

PS: The question ignores the fact that life is not possible on another planet. It is from pure astronomical viewpoint. On Neptune you have a different sky, so other questions/answers.


texts: How do you determine the apparent magnitude of the stars leaving main sequence with CMD diagrams?

texts:
So I have a CMD diagram and on the left, I have apparent magnitude and at the bottom, I have the color index. I would just estimate the magnitude of stars leaving the main sequence and continue from there, however, I was wondering if there's a way to do it more accurately?

I would take stars between certain magnitudes and calculate the average magnitude, but again, I'm estimating from where to where the MSTO point is...


texts: Rotational inertia, angular momentum of the Solar system

texts:
Is there Rotational inertia of the Solar system as one entity "disk"? Is this even possible?

The angular momentum of a multi-point system in the Solar system - Since the different planets have different mass and orbital periods then does create angular momentum, inertia in the system?

How can one planet affect another planet in the system from its individual rotation?

Does the solar system exhibit rigid body rotation from the mass distributed over planets?


texts: When would we detect a tiny meter size natural satellite in a geostationary orbits?

texts:
A natural, tiny (meters-size, maybe 10.000kg mass) natural satellite could be trapped in a geostationary orbit. I wondered for quite some time:

When and how are we able to detect these satellites?

I assume the distance of 35.700km of way too far for naked eye detection. So the next realistic opportunity would have been Galilei who first used telescopes for scientific observations of the sky. Could he have detected such an object?
Now even if he had enough resolution, he certainly didn't systematically cover the whole 4$\pi$ of the sky (especially as the satellite could be syncronous with the other side of the earth, and he could never see it).
Then, would it be at the times of Hubble (because of the Mount Wilson Observatory and other similarly powerful telescopes)? Would it at the times when quality equipment for hobby astronomers became cheap and therefore widespread enough (to cover huge areas of the sky)? Or would we - until today - not be able to detect such objects?
To answer this question, one needs to consider both the technical ability as well as the area of the sky that is covered.


texts: If a star orbited another star, how would that affect parallax measurements of the other star from the vantage point of the first?

texts:
Example:

If hypothetically, the vantage point would move 2AU along an orbit around the star being measured, would that make it seem as if the other star moved twice the distance over the back-drop?


texts: number density of Dark Matter Halos Versus number density of Galaxies

texts:
Can we count only one dark matter halo for each Galaxy ?

Indeed, I am trying to estimate the total density of galaxies in given volume and I have often seen on physics exchange and other forums the estimation of dark matter halos distribution (as a function of the mass of halo or as a function of Luminosity).

If I could count one galaxy per halo, this would make the things simplier but I think that things are more complicated, especially since a halo and the galaxy associated to this halo have not the same mass, so distribution mass is different (mass of halo is not linear towards the mass of galaxy, isn't it ?).

If someone could help me on this issue, regards


texts: Does anybody know how to get Sun-Jupiter Lagrangian points L4 and L5 RA and Dec?

texts:
I wanted to point at the L4 and L5 of Sun-Jupiter to find Trojan asteroids but I don't know how to calculate the RA and Dec of these.
Do you know of any software that does it for you or how to calculate them?


texts: Primordial supermassive black holes. How old are they?

texts: I know that astronomers have detected supermassive black holes in the early universe. What was the age of the universe at which these black holes were detected?  In other words how long is it thought to take for these supermassive black holes to form and what are the mechanisms by which they formed, other than from supermassive stars?


texts: What is the meaning of "bright unresolved sources"

texts:
In many articles talking about telescopes, they talk often of "bright, unresolved sources".

"bright" usually mean that the magnitude should be less than a certain threshold, (or that the signal to noise ratio should bigger than a given value) but what I don't catch the meaning of "unresolved".

For example :

"unresolved sources" in https://docushare.lsst.org/docushare/dsweb/Get/LPM-17/LSSTSRD_v5%202%203.pdf

"bright unresolved point sources" in the introduction of this paper : https://docushare.lsstcorp.org/docushare/dsweb/Get/Publication-86/

Any idea ?


texts:
According to Newton's laws, the trajectories for the two-body problem are conics: either ellipses, or parabolas or hyperbolas. Of course periodic motions require Ellipses and in the Solar system these elliptic curves are a pretty good approximation of the trajectory of each planet.

However, it is possible that the trajectory of a very fast comet could be an hyperbola or a parabola with the Sun at a focus; in that case the motion would not be periodic and the comet would be visible near the Sun only once.

My question: was there ever any observation of a  comet on a hyperbolic (or parabolic) trajectory (with the Sun at a focus)? Same question for different stellar objects. It would be certainly very difficult to observe, since the comet would be visible only one time at its perihelion and would go at infinity after this with no return.


texts: How can I calculate 27 tropical/solar year since my birthdate?

texts:
Let's say I was born on 27th November 1993 at 03:15:15 AM. Is there any way I can calculate exact 27 tropical cycle since my birthdate/birthtime.

What date and what time it will be after exact 27 tropical years since my birthdate/birthtime?


texts:
As an amateur astronomer for 60 years and always as interested in the science as in the observing, I thought I had a pretty good understanding of the fundamentals. But I'm distance-auditing a third-year course in the fundamentals of applied astronomy, and I found that I had a misconception about the color index.

I had assumed the CI was derived by measuring the star's own magnitude in each of, say, the B and V passbands, then subtracting to get the CI (e.g., by taking the ratio of flux intensities, expressed as a magnitude).

Not so! It's derived by measuring the star's magnitude relative to standard zero points (the magnitude of Vega at each of those wavelengths) that are defined to be magnitude 0 for those wavelength, then subtracting to get the CI.

My question is: Why measure two fluxes relative to a standard and then compare those two values, rather than simply measuring them relative to each other? Why not just measure the flux at each wavelength, take the ratio, express this as a magnitude, and use that as the CI? The relationships are the same either way, just with a different zero point (think 0K and -273°C). It seems like finding how far apart Cleveland and Chicago are by measuring the distance of each from NYC and subtracting, rather than just measuring the distance between them. I don't see that the convention accomplishes anything other than assigning a CI of 0 to "neutral" white stars. Or am I missing something?


texts: Can fish and other sea creatures survive in watery planets like Neptune or any other planets that has water?

texts: I'm a bit curious since Neptune does not have a land surface and it is covered with water, I mean water has oxygen right? What will happen if we put some sea creatures in these watery planets? Can they survive or live? I'm not referring to Neptune alone, I mean all the planets that has some large amount of water in it. Thanks.


texts:
When matter crosses the event horizon, it is easy to imagine that matter is torn apart into their individual components by tidal wave effects. Atoms would be ripped apart eventually.

At one time, the particles will fall so deep into the gravity well, that a single particle would be touching both sides of the gravity well at the same time. At that moment, I choose to assume that the particle particle core may be spaghettified, but externally it will appear to be shrinking.

Knowing that spaghettification is dependent on the size of the object, as well as the difference in gravity on both sides, a shrinking particle may eventually become so small that the steepness of the gravity well is no longer a problem.

The particle continues to shrink forever. Over time, gravity between all the particles that have shrunk, will again start to take effect - and I imagine that that is what happened in the big bang.

So the cosmic inflation, could just as well be particle shrinking, as a result of all particles in our universe being within the same black superhole.

Taking this idea, we could look at our own universe and I guess we would expect to see certain effects such as accelerating cosmic expansion, perhaps a cosmic background radiation and we would expect our universe to end as a cold, dead place.

Is this a known theory or idea, and where can I find papers on it?


texts: What measure should I use to help optimise the design of a telescope in a cubesat?

texts:
I have started a project in Melbourne, Australia called nanosat eye in the sky to put a telescope in a cubesat and put it in LEO. The idea is then to add the telescope to itelescope.

I need to learn a heap about designing telescopes. I have limited space and want to understand how to measure the theoretical performance of our telescope.

So my question is- how do I measure the performance of the telescope?


texts: How a star's rotation affects a star in the main sequence?

texts:
I searched and have found that the questions mainly focus on neutron stars, white dwarves and black holes. This was not what I looked for.

Basically, the bigger the star's mass is, the more intense is its fusion reaction and the shorter is its lifespan in the main sequence. Now, imagine that a star spins much faster. There will be less surface gravity at the equator than at the poles. How does a fast rotation affect the star's fusion reaction?

Will the pressure of the star's mass on its core be lower at the equator and therefore reduce nuclear fusion rate?
Will there be a deeper convection because of the Coriolis effect?
What shall we notice in term of longevity in the main sequence, luminosity and emission spectrum of a fast rotating star compared to a slow rotating one of the same initial mass?


texts: Is the size and heat of a star important in the development of a solar system?

texts: How do the size and heat of stars affect development of solar systems and what kinds of planets can develop? Is it possible to orbit supermassive stars? Furthermore, would star's unleashing violent winds such as wolf-rayet stars be candidates for solar system formations?


texts: Good textbooks for a descriptive astronomy course

texts:
Brief tale of woe

My department at a small public university in the US has decided to begin offering a descriptive astronomy general education course for the first time in a couple of decades and we have no institutional memory of how it was done before nor anyone on staff who's taught such a beast.

And I've been tapped to teach it.

Requirements

It's to be a one-semester course with three hours a week of classroom instruction and a three hour lab scheduled late enough that we'll have some dark sky early in the semester and more as the year goes on (schedule has it in Fall Semester). Expected enrollment in the neighborhood of 20 to 30. I won't have a teaching assistant unless I can rope a physics major into doing it somehow (we're an all undergrad department).

As a "general education" course mine will mostly attract students majoring in non-science subjects, biological science or human science. The students will have had enough math to know (in principle) how to isolate any variable in, say, the law of universal gravitation but many won't remember at first and some will resist; many will have chosen this class in an effort to avoid 'hard' classes like our gen ed chemistry, physics, or geology courses.

I'm looking of a textbook that touches on things like

Observational coordinate systems, familiarization with the night sky (constellations, I suppose), finding things
The Earth moon system, phases, eclipses, maybe even tides
Kepler's laws
The planets, moons and other bits and pieces in the solar system
The sun in particular and stars in general; including the birth, evolution and death of stars in a low math way
Some observational stuff; parallax, standard candles, HR diagram ...
A unit or integrated material on the planetary missions would be nice
Larger scale structure of the universe; a cosmology unit wouldn't be out of place; how much dark matter (or worse dark energy) can actually be done?
A unit on extra-solar planet hunting would be nice.
A unit on other wavelengths?
What else goes into a course like this?

Please include some detail on what is good about suggested texts and how well it matches with the course I've got to teach. What does it offer that's special?

Random detail

At this point we own a 8" scope (reflector) with a axial mount and motor drive but no automated pointing and two pairs of decent binoculars (I can probably buy more binoculars, but additional scopes are probably out of budget for this).

It's been about 20 years since I pointed a scope up with my own hands, and my only "real" astronomical experience was doing some programming for a CCD backed 14" remote-operated scope in the early 90s (we even managed to track a MACHO light curve and agree with the big boys, woo hoo science!).

The department head thinks that every gen. ed. course in our department should cover the scientific method and some thermodynamics which I think we can fit in.

I sincerely intend to include at least one field trip out to dark sky to just look up with adapted eyes. It's been too long...

The only question I could find on meta suggested that a sufficiently focused recommendation question would be accepted. If this is a no go, perhaps I'll pop into chat at some point.


texts: Why is Uranus able to support a regular satellite system?

texts:
Uranus has an obliquity of 98° which means that the mutual inclination between a satellite orbiting in its equatorial plane and the orbit of Uranus around the Sun would exceed the critical angle for Kozai oscillations. This would drive the satellites to high eccentricities which would likely be extremely damaging to the stability of the system, yet Uranus does host a system of regular satellites on near-circular orbits in its equatorial plane.

This suggests something is suppressing the Kozai oscillations for the regular satellites, so what is doing this? I suspect it is linked either to the non-spherical shape of the planet or gravitational interactions between the moons, but I'm not sure which would be a more relevant factor.


texts: Does a meteor's direction change between country or latitude?

texts: So here is the question me and a friend of mine from Norway we saw a meteor but que said he saw it in the opposite direction as me and I was looking from spain, does that make any sense? Or we should see it going the same direction? Thanks in advance


texts:
When historically did we realize that the Sun is a star, like all the others?

An answer was posted, then the question was put on hold. I would like to answer:
If indeed someone asserted that the Sun was a star 2500 years ago, that is rather humbling. That it took until about 100 years ago for it to be proven is even more humbling!

People level a lot of claims against science and scientists. But if a field of knowledge takes over 100 generations to prove a true statement, perhaps they have a right to criticize. So, I think that science needs to do a better job of proving its theories. Either that, or people need to have more common sense, not to recognize something that has been staring billions of people in the face for all time. Perhaps, we should all be a bit more humble about the state of knowledge, even in obvious areas?

Maybe if we had taken the people who proposed this fact, and the concept of atoms, seriously at the time, we would be on our way to other stars by now instead of still having only visited that big white thing nearby (forgot the name just now) and that not for the past 40 years.


texts: Simulation and its necessity in astronomical research

texts:
I was wondering if mainly everything in Astronomical and Astrophysical was dependent on Simulations then what are the educational requirements are necessary to qualify as the Simulation Expert.

What softwares are needed globally? Are all simulations run on a Supercomputer? Do astrophysicist require coding experience? And, is Knowledge of Programming a must?


texts: Could anyone recommend a good book about the basics of astrophysics?

texts: I am sorry if a similar question has been asked before. I look at other questions but was not entirely satisfied. So, I am a Psychology student who would like to know more about the basics of astrophysics. Could anyone recommend a good book about this. Also, I am particularly interested in black holes, so introductory literature on that is welcome too.


texts: I'm looking for a way to access the data used to produce these plots for instance. These plots show the spectrum of various Wolf-Rayet stars. I found a few websites where you have some informations on those stars here for instance, but I'm not able to find and download a file containing the spectrum data of the stars.


texts: How to Calculate TRUE geocentric longitude of Moon's Ascending and Descending Nodes

texts:
I have already computed the Mean and True geocentric longitude of the planets like Sun, Moon, Saturn, Jupiter etc..
Even I have calculated the MEAN geocentric longitude of Moon's Ascending and Descending Nodes
But can not Calculate the  TRUE geocentric longitude of Moon's Ascending and Descending Nodes
Can anybody help please. Thanks in advance


texts: Is there a formula to calculate the angular speed at which the Sun apparently moves around the Earth?

texts:
Is there a formula for how to calculate the angular rate at which the Sun apparently moves around the Earth?
I know it takes 0.25 degrees/min, but as I understand it depends on other parameters as well (such as how close a certain place to the equator, months of the year etc.), but I'm looking for a formula that makes it easy to calculate based on my location proportional to the equator and different time of the year (unless if I'm mistaken and it's constant)


texts: Does a parabolic trajectory really exist in nature?

texts:
It seems it is very difficult to have e=1 perfectly in nature. The final state (being captured or running away) of a celestial body with a parabolic trajectory,
is determined by minor perturbation?


texts:
The NPR news article and podcast New Telescope Promises To Revolutionize Astronomy updates the status of the "Large Synoptic Survey Telescope under construction on Cerro Pachón in Chile".

NPR's Joe Palca's piece includes sound bites from astronomers, including Caltech astronomer Mansi Kasliwal:

  PALCA: Kasliwal says although LSST will detect these events, other telescopes are better suited to study them in detail. So the plan is to send out an alert to other telescopes when LSST sees something interesting. Of course, that means the other telescope has to drop what it was doing, but Kasliwal says it will be worth it.

Isn't the LSST expected to "see something interesting" quite often?

Due to the size and scope of LSST's surveying capability which is due to it's huge corrected field of view and huge focal plane array and huge image processing and event detection capacity, there is a potential for a significant increase in the rate of alerts generated and sent to observatories and to astronomers' cell phones (one notable example of such things).

Question: Will LSST make a significant increase in the rate of astronomical event alerts? Has there been an estimate of how the overall rate will change when it comes online?

I'm wondering if astronomers will get woken up more often by their phones, or if some observatories will have to make the decision to promptly change observing schedules or not much more frequently.


texts: Do triple or more times reflecting telescopes exist?

texts: Refractors only use the length of the telescope once, reflectors twice, catadioptric telescopes like those of the Schmidt-Cassegrain design three times. Have telescopes been built that reflect the incoming light at least once more?


texts: How was Io not torn apart by tidal forces during its formation?

texts:
Jupiter's moon Io is arguably one of the most volcanically active bodies in the Solar System.  The reason, according to NASA's page Scientists to Io: Your Volcanoes Are in the Wrong Place is believed to be caused by Io being

  caught in a tug-of-war between Jupiter's massive gravity and the smaller but precisely timed pulls from two neighboring moons that orbit further from Jupiter – Europa and Ganymede. Io orbits faster than these other moons, completing two orbits every time Europa finishes one, and four orbits for each one Ganymede makes. This regular timing means that Io feels the strongest gravitational pull from its neighboring moons in the same orbital location, which distorts Io's orbit into an oval shape. This in turn causes Io to flex as it moves around Jupiter.

So, how did Io form in the first place, given the tidal stresses acting upon it? Does this suggest (and what evidence is there) that Io 'migrated' into its present orbit?


texts: What is a hard spectral state vs. a soft spectral state?

texts: In X-ray astronomy, the source is considered to be in the hard or soft spectral state. So what is the meaning of the hard spectral state? What are the soft state and hard state in spectroscopy?


texts: Could the solar system survive in a void?

texts:
If the solar system were suddenly placed in the middle of a large void... would everything still function as normal?

Would we even notice any difference here on Earth apart from the night sky?

...and speaking of the night sky, would that even look much different to how it is already? I imagine just less colourful and and more sparse, but seeing whole galaxies instead of individual stars?

Or would moving the solar system into a void just mean the end of the world as we know it?


texts: What made cooler temperatures suitable for atom formation?

texts: I have read in relation to Big Bang theory that after 300,000 years of big Bang temperature was reduced to 4500 Kelvin and this gave rise to Atomic matter.So,my question is why reduction in temperature makes it suitable for atom formation?


texts: I know that the expansion of the universe is speeding up, but is there an equation to measure this? Also, what is the cause of this?


texts: Is the motion of the Sun around the Galaxy a result of gravitational pull?

texts: Does the orbit of the sun (and other stars within the Milky way) around the galaxy result directly from radial gravitational force toward the objects in the center of the galaxy, or is there additional forces at play?


texts:
I know that M (red dwarf) main-sequence stars slowly fade to white dwarf stars, and that G (yellow dwarf) main-sequence stars become red giants and then shed their outer layers in a planetary nebula leaving behind a white dwarf, but do K (orange dwarf) main stars take a similar route to red dwarfs? Or do they expand into red giants like yellow dwarfs?

What about F (yellow-white) main-sequence stars? Do they turn into supergiants and go supernova? Or do they take a similar route to yellow dwarfs? And what about A (white) and B (blue-white) main-sequence stars?


texts: Is the light we see from stars extremely old?

texts:
Our nearest star Proxima Centauri is 4.243 light years away from Earth.

Does that mean we are seeing light that is 4.243 years old everyday?


texts: Confirm: We can only notice planets if their orbit passes in front of their star and our view

texts:
If we can tell a planet exists by the dimming of the brightness of a star when it passes in front of our view of the star does that mean we cannot see a planet if it does not cross in front of our view?

If planet of a distance star goes around the star rather than in front of it than we cannot see that planet. Correct?


texts:
What are the axioms (if any) behind the LCDM model of cosmology?
NB: axioms, not postulates (e.g., inflation)


texts: What is the line of light in galaxy cluster MACS J1206?

texts: I noticed this in a thread from world building, what is the continuous line of light in MACS J1206 slightly right of center in this picture? The only thing I could think of would be the side view of a galaxy, but it's much longer than any other galaxies pictured.


texts:
Could someone explain what is "median stellar mass" and how is it calculated?
I have seen it written as:

$$\log(M)$$

or as:

$$\log\left(\frac{M_*}{M_{\text{Sun}}}\right) $$

but I do not understand what it means.


texts: How to convert magnitudes to bolometric luminosity?

texts:
If we know a star's magnitudes from several bands(say u,g,r,i,z,J,H,k) and the star's distance, how to calculate Lbol?

Let's suppose the star has a black body spectrum. Is there any manual about the calculation? It's better to have a lot of technical details.


texts: Is Blue Shift just as provable as Red Shift?

texts: I've heard a lot about red shift and it sounds pretty legit. But it made it sound as if blue shift was a lot less likely. Can anyone help me out?


texts: How much heat is generated from waxing and waning of reflected radiation from the Sun?

texts: According to this article, the greatest cause of global warming would be from the waxing and waning of reflected radiation from the Sun. (Placing carbon emissions at 10-20%) So really, how much heat is generated from the process that stays on Earth? And would it really affect it that much?


texts:
So Sky Safari has shown that comet 289P/Blainpain has had a sudden increase in brightness, and is currently sitting at mag -1.4!!! Last week it was sitting at +15. It is due to reach Perihelion on 20th December (2 days time) so is this due to some outburst? Or is there a glitch in Sky Safari? Looking at THIS site, it states:

  When the perihelion distance is larger than 1.0 A.U., it becomes very
  bright if it passes the perihelion on a special day in early December.
  However, only if the perihelion passage shifts by a several days.

But the Perihelion distance is 0.089AU for this year according to Google

This comet could be prone to outbursts as it was recently rediscovered in July 2013 due to a sudden brightness increase from an outburst (although nowhere near this much!)

However, currently TheSkyLive is showing that it is currently at mag +18.

I have had a quick search to try and verify this, unfortunately, I live in England and our skies are very cloudy at the moment so I can't go out and see for myself. Does anyone have any information as to whether this is a Sky Safari glitch or if not, what has happened to cause such a huge brightness increase?


texts: Neptune time of perihelion/ephemeris dates

texts:
Is there a data online for the time of perihelion of Neptune between 2000AD and 3000AD? I need values in Julian Date (JD).
I tried using HORIZONS Web interface, but it won't generate ephemeris for Neptune after a certain calendar date.
Thanks.


texts:
So I am a newcomer to Astronomy - only aged 15 - and am currently looking upon the sky in hopes of witnessing several meteors. This is my first time ever - maybe - seeing them. I just went inside for food and drink, come out and i saw a trail (this trail was not here before I went in). I do have a few doubts, one it was seen below the moon, when it was supposedly meant to be near Vega - however I read about them having a tendency to be different to predictions. Also, considering it is so close to the Moon and I can't currently see Vega from this position I highly doubt it.

please view the below image in the hopes of identifying the object (please excuse the camera quality, I am currently setting up the DSLR and this was photographed on my Nexus.)


texts: We know the existence of dark matter because we can test its gravitational mass (e.g. in gravitational lens) but, since we cannot see this matter, how we can be sure that it has an inertial mass, and it is the same as the gravitational mass? In other words, does the principle of equivalence apply also to dark matter?


texts: What does Sa and Sab mean for spiral galaxies?

texts: I have learned that the notation to classify spiral galaxies in the De Vaucouleurs system goes like "SA for spirals without bar, SB for spirals with bar and SAB for intermediate spirals". But in some places I've seen galaxies classified as Sa or Sab for example. What it means and what is the difference between a Sa and a Sab spiral?


texts: If we've only observed Oumuamua as a single pixel, how can we know its shape?

texts:
On a separate Oumuamua question (Oumuamua alien space junk?), I was confused how shape could be inferred from a single pixel, per another user's (@Uwe) comment. It was suggested I ask this question here.

If all we've observed Oumuamua as is a single pixel, how can its elongated shape be deduced from that? How can any non-uniform shape be deduced from a single pixel?


texts: How can a supermassive black hole be 13 billion years old?

texts: A supermassive black hole was discovered recently that is 13 billion years old. This blackhole is 1.6 billion times the mass of the sun. How can this blackhole have formed so quickly after the big bang? Do theories predict supermassive black holes forming so quickly?


texts: Are there any ways in which the light time difference between the center and the edge of the solar disk is noticeable?

texts:
In the ABC News video New space discovery has astronomers buzzing at about 02:30 Dr. Hakeem M. Oluseyi explains that the shortness of FRBs (before dispersion) suggests a compact source size and illustrates this by using the Sun as an example.

I'd never though about this until just now, but the radius of the Sun is indeed about 2.3 light-seconds. It takes light from the edge of the Sun's disk over 2 seconds longer to reach us than light from the central area.

Are there any ways in which this light time difference is noticeable, or needs to be taken into account?


texts: Why 2014-2015-2016 were the years when the number of discovered exoplanets increased the most?

texts:
So, if we look into the cumulative number of exoplanets discovered by year, you can appreciate a huge rise in the number of discovered exoplanets in the years 2014 and 2016:

Credit to NASA Exoplanet Archive
Also, I don't know if it's by coincidence or not, but the number of publications with the word "exomoon" (a satellite of an exoplanet) seems to increase dramatically by 2015:

Credit to app.dimensions.ai. I might be wrong, but I thought that we would see this rise in the years following 2010, like 2011 or 2012, when the Kepler telescope was beginning this hunt, and in the case of the publications, Kipping (2009a, 2009b) were very important around that time too, so is there any reason of why 2014-2015-2016 seem to be the most important years?


texts: how to calculate the half-mass radius and tidal radius of a (simulated) globular cluster

texts:
I am exploring a dataset of a direct N-body simulation of a star cluster, such that all particles have the same mass and

  G = M = −4E = 1

for the whole cluster. The data available is the positions and velocities of all stars (at certain snapshots of the simulation).

I'd like to be able to calculate some basic characteristics of the cluster. First I did the center of mass (which is just the mean of all coordinates since all masses are equal), but I am not sure how to proceed for these two:

The half mass radius - I can't seem to find an established (i.e. most efficient) way to calculate this. My first idea is to use a bisection method with the center of mass and the total radius as starting points, and do window queries with a spherical envelope until it contains half the stars. Is there something more efficient than that?
The tidal radius - what I've found so far always involves variables drawn from the galaxy the cluster is in (like the total mass of the galaxy and so on). However, this being a simulation, there is no such data. Yet the dataset states that some stars escape the cluster over the length of the simulation... I must be missing something here


texts: How strong are the tides raised by Io on Jupiter relative to the ones raised by the Moon on Earth?

texts:
There seems to be 2 ways of calculating tidal forces that appear contradictory. Either:

By calculating the difference of Io's gravitational acceleration on a point on Jupiter's near side and the gravitational acceleration felt by a point on Jupiter's far side, using the equation:

$$\frac{GM_{io}}{(d-r_{jupiter})^{2}}-\frac{GM_{io}}{(d+r_{jupiter})^{2}}$$

For the Io-Jupiter system, this is $24.0807\times10^{-6}$ Newtons, while for the Moon-Earth system it is $2.2009 \times 10^{-6} $ Newtons.

So if you were to calculate tidal forces that way, the tides raised on Jupiter by Io would be $10.94$ times as strong as the tides raised by the Moon on Earth

However there is also another way I found, which is:

By using the derivative of Newton's law of universal gravitation, using the equation:

$$\frac{-2G\times M_{jupiter}\times{m_{io}}}{d^{3}}$$

For Io, this is $3.015 \times 10^{14}$ Newtons per meter, whereas for the Moon it is $1.031 \times 10^{12}$ Newtons per meter.

So if you were to calculate tidal forces that way, the tides raised on Jupiter by Io would be $292.5$ times as strong as the tides raised by the Moon on Earth

So, which one is it? Are the tides raised on Jupiter by Io 10.94 or 292.5 times as strong as the ones raised by the Moon on Earth? My intuition leads me to believe the first answer is correct since it takes the primary's radius into account but I just want a concise answer.


texts: At about 4.15 am this morning a light travelling in a southerly direction passed over Adelaide South Australia. It was travelling 10 - 15 times the speed of an Airliner. The light was constant and moving in a straight line


texts: Hypothetical trans-Neptunian planets orbiting the Sun

texts:
The planet HD 106906 b was found at 650 AU from its home star, a star not so different from the Sun. Fomalhaut b is also a young planet that is orbiting very far away from its star.

These two planets were discovered mainly because they are young enough to still be hot enough to be seen in the infrared. I think that old planets in very high orbits would have temperatures below 10K, and thus be black and invisible.

Some people agree with the hypothesis that, in our Solar System, somewhere out beyond Neptune, some non-dwarf planets might be orbiting the Sun. The existence of HD 106906 b and Fomalhaut b suggests that this is possible. What are the odds that we could find at least one of these planets in our Solar System? If they do exist, how could we find them?


texts: Where can I look up assigned origins of gamma ray bursts events? (GRBs)

texts:
Gamma ray burst GRB170817A is believed to have originated from the merger of Neutron Star(NS)-Neutron Star(NS). From this article on the associated gravitational wave detection GW170817:

The gravitational wave signal, designated GW 170817, had a duration of approximately 100 seconds, and shows the characteristics in intensity and frequency expected of the inspiral of two neutron stars. Analysis of the slight variation in arrival time of the GW at the three detector locations (two LIGO and one Virgo) yielded an approximate angular direction to the source. Independently, a short (~2 seconds' duration) gamma-ray burst, designated GRB 170817A, was detected by the Fermi and INTEGRAL spacecraft beginning 1.7 seconds after the GW merger signal. These detectors have very limited directional sensitivity, but indicated a large area of the sky which overlapped the gravitational wave position. It has been a long-standing hypothesis that short gamma-ray bursts are caused by neutron star mergers.

Are there lists or compilations or reviews of all reported gamma-ray bursts (GRBs) together with an assigned or hypothesized event or source that produced it?


texts: Is there any planet or black hole where we could experience time dilation like in the movie Interstellar?

texts: As we seen in Interstellar movie Copper goes to a planet where he experienced time dilation which was 1 hour of staying on that planet will cost the 7 years on the earth. So, is there any planet or black hole where time will nearly behave like this? Because the planets in our solar system will show very little differences.


texts: Has anyone filmed a total lunar eclipse from the Moon's perspective?

texts: With all of the footage we have of total lunar eclipses, has NASA, ESA, or any other space program ever taken a photo or video of it from the Moon itself?


texts:
In this photo taken by SpaceX during one of their Falcon 9 launches, I noticed a familiar group of stars but I'm not sure of what it really is. I encircled it in red, is this group the Cancer constellation?

Original Image


texts: Time dilation on an object circling earth

texts:
How would a receiver on Earth hear a radio transmission from an object circling earth at 99% of the speed of light for 24 hours. The transmission from the object circulating would be non-stop.

Because time dilation would occur, would the transmission be slowed to the receiver? The object would travel so near earth that the signal would not have any delay reaching earth.


texts: Focusing light through fiber optic cable to extend telescope viewer

texts:
I want to build a system to view my telescope without being uncomfortable. Currently I have to stand up and bend over to view the eye piece.

I want to build a system that allows me to 'pipe' the output of the view piece in a non-digital format along a flexible fiber optic cable to my eye and possible split the view into two for a sort of VR headset.

I know the cable will transfer the light, but will it be just burly light or break down too much (one meter or less) or will I be able to focus it into view-able picture?

I could just try it, but the cable itself is really expensive to get, and I want to know if this is worth trying before I spend.

I'm thinking of using this 14 mm 'solid core' cable


texts: What would a 220km diameter asteroid impact do to Earth?

texts:
I was looking at this chart,

which shows that an order of magnitude increase in asteroid impactor diameter roughly corresponds with two orders of magnitude increase in it's rarity.  So if the K-T Impactor at 10km diameter is a 100 million year rarity impact, then a 100km diameter impact has a 10 billion year rarity (on the order of the lifetime of our planet).  On a large time scale, orbital perturbation allows for some chaotic orbits, so it might not be vanishingly likely that one of the big asteroids eventually hit earth. The asteroid 16 Psyche orbit overlaps in distance to the sun with Ceres, so it could potentially get a natural gravity assist into another orbit.
What would happen to Earth if it got hit by the asteroid 16 Psyche at, say, 40 km/s in the Pacific off the coast of Japan?
I am specifically looking for an answer to one or more of the following questions: How big would the crater be?  How much kinetic energy would be released?  What portions of that would be transferred into heat, ejecta, and deformation?  Would there be a secondary antipodal mountain formation like the Caloris impact on Mercury?  Would it cause magma oceans?  Would life on earth survive to reclaim the surface before the Sun went red giant?


texts:
I'm a Machine Learning Student and I'm working on Strong Lensing Image Classification. this is a hot topic nowadays but strangely after days of searching and communicating with different people I still can't find an accessible Dataset of these objects!

Do you know any place I can find a simple(or not) Dataset containing images, labels and some extended features of Strong Gravitational Lensings?


texts:
As I understand, the evidence for dark matter is the observation of the gravitational dynamics of objects around the size of galaxies and above.  They move in a way such that the equations imply there is more mass than what we can see.  There has never been any direct observation of dark matter, and our theories tell us it cannot be composed of any elementary particles from the standard model.  Furthermore, there is no observation which suggests that dark matter is unevenly distributed, that some galaxies consisting of roughly the same amount of normal matter have varying amounts of dark matter.

I wonder if therefore an alternative explanation for the phenomena is that we have the wrong dynamical equations for gravity.  Just as Newtonian gravitation is a good approximation to general relativity at low masses/energies, perhaps general relativity is a good approximation to the true gravitational theory at sub-galactic distances.  Maybe when distances become very large we start seeing that gravatational force is described by slightly different curves than what general relativity predicts.

Is there any good evidence speaking against this alternative approach?

The reason this question is different from the "possible duplicate" is that the other question asks if the source of the effect attributed to dark matter is parallel universes, rather than different dynamical laws.  However, the answer there does mention these alternative gravitational theories.

I realize now that this is not a new idea, and I thank the commenters and answerers for pointing me to existing research.  However, this part of StackExchange does not bill itself as an "experts only" forum, so I don't think the downvotes are fair.  In mathematics for example, there is one forum for experts, and another for everyone.  I'm an expert in something, but not astronomy.  Thanks.


texts: Has anyone ever tried to make a simple telescope using ice?

texts:
I grew up with long cold winters, and saw a lot of remarkably transparent ice formed by refreezing meltwater, both in puddles and ponds, and in large icicles. I'd always thought about making optical elements but never tried, as I knew getting a good surface figure would be quite a lot of work.

After clicking through links in this answer to the question Are there any natural materials that can be used as a magnifying lens (or to craft one)? I came upon the images below, which have rekindled my interest.

However I'm currently near the Tropic of Cancer and far from any "Winter Wondernalds".

So I'd like to ask if anyone has ever tried to build a simple, low magnification telescope from ice or if there are any references to such an attempt.

I don't care if it's a front-surface reflector for the Sun or a refractor for astronomical or terrestrial viewing. I'd just enjoy seeing any, even slightly successful attempt at getting some optical magnification from surfaces produced in ice. Conventional eyepieces would be fine, as would projection on a piece of paper.

Some related imagery to "get the ball rolling":

From http://wildwoodsurvival.com/survival/fire/ice/ist.html also see Fire from Ice

From http://wildwoodsurvival.com/survival/fire/ice/istmake.html

From http://www.primitiveways.com/fire_from_ice.html


texts: Effect of the obliquity of the ecliptic / tilt of the Earth on the equation of time

texts:
In an attempting to answer the question “why aren’t changes in sunset and sunrise times symmetrical  around the solstices ?” I’ve got stuck on being able to explain the effect of the obliquity of the ecliptic.

I understand the general idea of the equation of time and how the difference in clock time and solar time answer my original question, but I’m struggling to grasp how the tilt of the Earth contributes to the equation of time.

I’m trying to visualise the effect, and assuming it is to do with the point at which the sun is highest in the sky not being in the middle of sunset and sunrise (except at solsitces and equinoxes?). So, is this correct? Is there any good way of trying to visualise it ?

I’m a bit confused by the wikipedia entry
https://en.wikipedia.org/wiki/Equation_of_time#Obliquity_of_the_ecliptic saying

“If this effect operated alone, then days would be up to 24 hours and 20.3 seconds long (measured solar noon to solar noon) near the
solstices, and as much as 20.3 seconds shorter than 24 hours near
the equinoxes”
“the inclination of the ecliptic results in the contribution of a sine wave variation with an amplitude of 9.87 minutes and a period
of a half year to the equation of time. The zero points of this sine
wave are reached at the equinoxes and solstices”.
Is point 1 correct ?

Also found these links helpful to some extent, but haven’t fully explained it for me.
https://web.archive.org/web/20150910174438/http://www.rmg.co.uk/explore/astronomy-and-time/time-facts/the-equation-of-time
http://totaleclipse.eu/Astronomy/EOT.html


texts: What causes fast moving pulsars to move so fast?

texts:
This article investigates the traces left in the ISM by fast moving pulsars. Is there a mechanism specific to pulsars that causes them to move so fast, or are there just as many fast moving stars?

The article also mentions that

  Many fast moving pulsars quickly escape  the  host  supernova  remnant

This seems to indicate that something during the supernova makes the pulsar speed-up (or the remnant slow down). How does this happen?


texts: What fraction of a star's hydrogen store will be fused over its lifespan?

texts:
A main sequence star will fuse some of its hydrogen, but not all. In massive stars ($>1.5M_\odot$) the core is convective but the rest of the atmosphere radiative and hence does not mix much: as it undergoes shell fusion it will produce an onion-like structure with unused hydrogen on top. Solar mass stars only do this up to helium, but again leave a mantle of unused hydrogen. Stars less than $0.35M_\odot$ are fully convective and can in principle use up all hydrogen. However, I suspect this is not complete except for very low-mass M dwarves that have trillions of years to mature.

Looking at planetary nebulae, I have seen statements that they are about 90 percent hydrogen, 10 percent helium. This seems to fit with papers I have found (example, example) although in some cases helium may reach 29% (example). Given that for a G star about half of the mass is ejected, that would suggest a fraction $0.9\times 0.5 = 0.45$ of unused hydrogen. But surely the fractions will be different for other masses.

So, to sum up, what is known about the fraction of hydrogen that is never fused over a stellar lifetime?


texts: How to Distinguish between Temperature and Doppler effect using Black-body radiation?

texts:
I understand that the radiation of a body can be described using the curve for black-body radiation. In the sense that a hotter body will be blue shifted and a cooler body will be red shifted.

The doppler effect gives similar results in the sense that a body is approaching us is blue shifted. While an object that is travelling away from us is red shifted.

How is it possible to distinguish between temperature and doppler effect?


texts: How astronomers distinguishes between natural and artificial signals coming from outer space?

texts: How astronomers really distinguishes between the natural and artificial signals? From time to time astronomers detects many signals from outer space and SETI is always trying to find signals that carries the sign of intelligent beings. The Wow signals was such a much hyped signal that we received. So what properties a signal should carry so that we can conclude it is from some alien civilizations?


texts: How many generations of stars can be formed in the Stelliferous Era?

texts:
As in general case, stars are formed from nebula which in some case itself is the result of a supernova.
Also stars need hydrogen to become a star (to do nuclear fusion), but stars consume hydrogen in their lifecycle. Thus, less and less hydrogen is left for the future generation and more heavy elements take that place.
How many generation of stars can be formed until there is no more enough hydrogen left to create new stars?


texts: Why is the storm on Jupiter much stronger than storms on Earth and why don't the storms on Earth get as big as Jupiter's great red spot


texts: How can cosmic inflation make an infinite universe homogeneous?

texts:
As it is explained in this video, one of cosmic inflation's observable effects is the homogeneity of our universe. Inflation allows two points on the different sides of the observable universe to be causally connected at some point in time, so they may exchange their mass densities and temperatures, which are then going to be about the same for both of them.

While I do understand, that this effect doesn't have to end on the edge of the observable universe and might go on for whatever distance the rate of inflation will make it to, I do not understand how this could be valid for the entire Universe if it is infinite. From my guessing, if the Universe is infinite, there will allways be two points, that was never causally connected, no matter how fast it expanded at the inflation period. That will mean, that, while the Universe should be homogeneous on the sufficiently large scales, it doesn't have to on the scales even larger.

I guess I either don't fully understand the idea of cosmic inflation or don't get the particular way the Universe is infinite in.


texts: How to find the mass of a planet not knowing the gravitational constant?

texts:
So I found this problem, I know the gravity of earth $g_0$, I found the orbital speed of one satellite using this equation $\frac{GM_em_s}{(R_e+h)^2}=m_sa$
($M_e$ is the mass of earth, $m_s$ is the mass of the satellite, $R_e$ is the radius of earth, $h$ is the height of the satellite)
and by taking $h=0$ I found the gravity of earth $\frac{GM_e}{R_e^2}=g_0$ so I found $GM_e=R_e^2g_0$ and then I found the orbital speed $v=R_e\sqrt{\frac{g_0}{R_e+h}}$ and then I calculated kepler's third law. but the last questions says deduce the mass of the earth $M_e$, How can I do this by not knowing the gravitational constant?
I'm sorry for bad english, and thanks a lot


texts: What would happen if a neutron star merged with a white dwarf?

texts: We've heard of neutron star mergers and white dwarf mergers. But what would result in a neutron star merging with a white dwarf? Would there be a similar super/kilonova, are there any examples of it, and what would the remnant/merger be?


texts: Why do we have the cosmological constant?

texts:
Since the cosmological constant is not required to explain that the universe seem to be expanding, why do we have it?

What other factors cause us to have that constant?

Background:
Without the cosmological constant, distant stars should be affected by a great redshift. The amount of redshift is a function of their distance from us. This is due to gravitational time dilation. We are looking 13 billion years into the past, where the universe was very dense. Those stars should be experiencing extreme gravity, causing Einstein shift.

Since we DO have the cosmological constant, we are now looking for other explanations for the redshift.


texts:
Hi built my own newtonian telescope for the first time. Up until that point I had never used a telescope.
Can you please look at the photos attached and tell me if this is what Mars should look like as I'm disappointed somewhat.
Specs are as follows:

Picture taken with smartphone through the eyepiece and no filters were used or edited.
TMB Planetary Series 2.5 mm 3.2 mm eyepieces
10" (25 cm) Newtonian with f = 1470 mm

uncropped original


texts:
I've travelled to farms at night, and away from civilation. I don't really know alot about astronomy, but my goal is to see something like this:

Stars, lots of them, but that "thick purple cluster of gas right in the center". I want to see that.

I assume that on a very very dark day, we would be able to see this, but sometimes I only see stars, just lots and lots of stars, something like this:

I want to see this "thick cloud cluster, which I assume is the milky way". How do I see this? How on some days do I only see just stars and stars, when I really want to see this main beauty?


texts: Magneto tail effect on the other planet in a binary planet system

texts:
Let's say there are 2 earth sized planets orbiting each other around 750,000km apart and at a distance of roughly 1.2 AU from a sunlike star with a mass of 1.105 solar masses. Since they are binary, they would most likely be on the stellar plane and therefore should eclipse eachother regularly.
Both planets have their own rotation and their own magnetosphere similar to Earth's.
As one planet orbits in between the second planet and the star, the Magneto tail would pass over the second planet.

Question: What would this do to the other planet?

Would the second planet handle this the same as it would the regular stellar wind? Would it blow concentrated radiation at the second world ?


texts: How do magnetars create/sustain such strong magnetic fields? If a dynamo effect did create a magnetar, then what creates the magnetic field? The dynamo effect requires an electrically conductive fluid, but magnetars are purely composed of neutrons. In Earth, the dynamo effect is supported with evidence, but how does it apply to magnetars?


texts: Why do astronomers like green laser pointers?

texts:
When I've been to Astronomy-nights for the public at the local observatory, they always used green lasers to point things out.

Why is there such a preference for green lasers?

What's wrong with red lasers? (Which seem cheaper and more readily available.)


texts:
On July 13, 2018, while camping in Southwest Oregon, a bright light appeared in the sky at sunset. It was much brighter than Jupiter, also visible. It was moving slowly, west to east. I estimate it took 1/2 hour to cross the sky, though I didn't watch it all the way. The brightness seemed to vary, as did the coloration. It seemed it was reflecting the sunlight. It was higher than the clouds.
   I had a spotting scope with me and it was not a point nor a disc, but rather circular, bright around the edge with a less-bright center. It kind of looked like a planetary nebula, only quite bright. I thought maybe it was a weather balloon, but I really have no idea.


texts: Can you create an orbit in a space station using balls?

texts: Would it be possible to spin a ball in a space station, and for it to create its own orbit for a smaller ball to rotate around it?


texts: What is the longest natural bound orbit chain observed?

texts:
Define a bound orbit chain as a list of successively less massive bodies, each in a bound orbit with the bodies preceding it in the list.
Then an example of a bound orbit chain would be: <Sun, Earth, Moon>, since the Earth orbits the Sun, and the Moon orbits the Earth and the Sun (and they are listed from most to least massive).  This is a chain of length 3.
Question: What is the longest natural bound orbit chain we've ever observed?
Related: Do moons have moons?


texts: At what speed does something have to travel away from us for it to red shift enough that it becomes invisible to the human eye?

texts:
Are there stars, galaxies etc that we cannot see because they are traveling too fast and their spectrum is shifted below our visible range? From what I understand, red shift is caused by stars and such moving away from the viewer. At what speed does a star etc have to travel to be invisible to us?

What if you are in one galaxy on one side of the universe that is expanding looking at a galaxy on the other expanding edge side. They would be traveling faster than the speed of light away from each other. Would they be invisible to each other?


texts:
Brown dwarfs (BD) are often depicted with stripes.

        brown dwarf                              Jupiter

Pictures of BD resemble Jupiter but brown dwarfs have even more stripes.
Question:
How do we know brown dwarfs have so many stripes?
I suppose it can differ depending on BD's temperature and mass. Probably only cold BD can have stripes.
Were the stripes observed? (by photometry, by transit or occultation, etc)
Or are the stripes result of theoretical modeling? Or just "artist impression" because of lack of information?
If result of modeling - do the stripes represent analogs of Hadley cell athmospheric circulation? Are they high and low pressure bands?

Do brown dwarfs have so many bands because of higher gravity?


texts: Seeking advice regarding buying a telescope

texts:
I'd like to buy a telescope with a focal length of 700mm and an aperture of 60mm.

Would I be able to use a telescope of this power to study the closer planets (for example Mars and Venus) and nebulae?


texts: What methodology should be used to choose the correct model for origin of QPOs in X-ray astronomy?

texts:
In X-ray astronomy, quasi periodic oscillations (QPOs) are routinely shown by black hole and neutron star X-ray binary systems in their X-ray flux. Despite being strong and easily measurable signals, their physical origin is yet unknown. Several models has been proposed, in particular for the low frequency (LF) QPOs, ranging from well-developed theories that have been confronted with data to ideas briefly described in a few papers.
Most of the models that are confronted with the available data try to explain the phenomena using their model. But ultimately the astrophysics community should choose a particular model as the correct description for the origin of the QPOs. In this scenario, what needs to be the criteria that would decide which model is the correct model?
N.B.: I am not asking for a discussion on QPOs, rather I am interested in the methodology that is used to choose the correct model for QPOs.


texts: singular star moving in the sky, can’t be satellite nor shooting star

texts:
so i was just setting up my camera for some long exposure photography and just as i was about to take the picture, a star seemed to be moving in the sky on its own. there was no contrail like a shooting star would have, and there wasn’t any obvious flashing like a satellite would have. it just moved quickly across the sky at a constant speed. and this isn’t the first time i’ve seen this either, i’ve seen it two other times.
what could it be?


texts: How were the designations of "North" and "South" applied to the hemispheres of Mars?

texts: How was it decided which hemisphere was North and which hemisphere was South on Mars? Clearly, the designations "North" and "South" apply only to the hemispheres which contain the axis of rotation at their center, but what determined whether a hemisphere received "North" or "South" as its designation?


texts: Is it accurate to say that we have a gravitational attraction towards all objects in the known universe?

texts: Its probably small, but is there a theoretical gravitational attraction between all objects in the universe? Light can move pretty far, so does that mean gravity can as well, and is the gravity blocked by the same way light is blocked by an object?


texts: Has the effect of other planets on Earth's orbit been ever measured?

texts: Since other planets have their own gravitational pull, I'm wondering if their effect on Earth's orbit has ever been measured? I know that in comparison with the Sun's pull the effect might be rather small, but I'm wondering if it's measurable...


texts: Stars in constellation patterns by catalogue number?

texts:
I need a resource that can tell me the stars that make up a constellation (pattern), preferably by Henry Draper or some catalogue number? All i can find is lists of every star in a constellation (area) or incomplete lists of stars by name.

If this doesn't exist can you suggest a way to work it out myself?


texts: Question from a non-astronomer /non-mathematician regarding rotation speeds

texts: I have been watching too much TV related to black holes.  Most black holes are depicted with a disk of matter that spirals into the black hole. Does the mass and diameter of the disk have a bearing on the rotational speed, maximum rotational speed of a black hole? Also, do black holes have gravity? It looks like they must in order to collect matter. Does the gravity of a black hole create gravitational waves much as the electromagnetic waves created around the earth?  Do these waves have any influence on the speed of the black hole's rotation. What about if the rotation if the black hole is not symmetric?


texts: Anatomy of a galaxy cluster: understanding the relationship between dark matter halos and subhalos and galaxies within the clusters?

texts: In my understanding, galaxy clusters contain dark matter halos, which in turn contain subhalos, and the subhalos are what surrounds individual galaxies. I know that is a simplified picture, but I am just trying to figure out if it is correct in general. Additionally, when people talk about "substructure," does that apply to halos or to subhalos (or sometimes both)? My question stems from reading different papers where these terms seem to be used in slightly different ways. For example, here they talk about substructures as being individual subhalos, but I have heard the term in reference to all structure smaller than the cluster itself.


texts: why there is a circle of light surrounding Sun and Moon?

texts:
One day when I was playing basketball I saw a circle around the bright Sun. It is a perfect circle with a radius of about 20-30 km.

Why does this circle of light form? I have seen the same also around the moon. But this does not happen every day. Why ?


texts: If one of the rules to be a planet is that it needs to clear ALL objects from their orbit, does this also make Neptune a non-planet? Since it has thus far failed to clear Pluto (and based on an answer from the physics board, the kuiper belt) from it's orbit. Or does this rule not really apply to planets and we should welcome Pluto and a few other dwarf-planets into our family of planets?


texts: Relativistic effects in stellar dynamical systems

texts:
I am curious, if anybody knows of any stellar dynamical systems/environments, where relativistic effects could play a dynamical role on the motion of these stellar systems? As a subquestion - are there any known important weak, but cumulatively strong effects?

In other words, when can relativistic effects invalidate the applicability of N-Body/Collisionless Boltzman/Gas/.. models based on newtonian gravity.

From these systems I would like to exclude the simplest well known case of compact binaries.


texts: If our sun was already in the process of becoming a red giant, would the gradual rise in luminosity be noticeable to our eyes at some point in human existence?


texts: pp-Chain reactions involving $^{3}_{2}He$ - differences in S-factor

texts:
So I was looking at the pp-chain reactions that take place inside stars in a bit more detail.
I got confused about the massive differences in reaction efficiencies concerning two reactions $^{3}_{2}He$.
In the lecture notes I've googled (the ones that I've understood best so far), the reaction
$$^{3}_{2}He + ^{3}_{2}He \rightarrow p + p + ^{4}_{2}He$$
has an efficiency factor $S_1 \approx 6MeV-barn$ and being quoted as 'strong reaction'.

While the other reaction I'm confused about in contrast is
$$^{3}_{2}He + ^{4}_{2}He \rightarrow ^{7}_{4}Be + \gamma$$
with a way lower reaction probability of $S_2 \approx 4\cdot 10^{-4} Mev-barn$ and being quoted as 'electromagnetic interaction'.

So my question: How can we understand the huge discrepancy between $S_1$ and $S_2$? Particularly, when is a reaction a 'strong' interaction?
Isn't the strong force necessarily always involved, else there wouldn't be a potential minimum in the nucleus to tunnel to...

Note: The S-Factor is just a part of the velocity dependent reaction cross-section $\sigma(v)$ so that $\sigma(E) = \frac{exp(-\tau)}{E} S(E)$. Here $\tau$ is the tunneling probability from Quantum theory.

I would also appreciate any good textbook reference, as so far I'm working with google and random lecture notes I'm finding.

Edit:
Thoughts on this:
In both cases particles must tunnel towards each other first. Also we know that $^{4}_{2}He$ is a bound state for the nuclear force.
So what might be happening is that in $^{3}_{2}He + ^{3}_{2}He$ the nuclei rearrange themselves into the energetically favoured bound state, kicking out the surplus protons.
In the case of $^{3}_{2}He + ^{4}_{2}He$ the bound state already exists, and thus in most cases the reaction will be reversed. Only a few of those reactions (why?) allow the spontaneous formation of a higher-shell object like $ ^{7}_{4}Be$, thus the much lower reaction rate.
If this story so far makes sense, then I still don't understand when and why  $^{7}_{4}Be$ is formed, I'd be grateful for explanation.


texts: How was Earth's "quasi-satellite" 2016 HO3 "first spotted" and it's orbit determined?

texts:
Under a question I asked a few days ago Have there been any documented mini-moons since 2006 RH120? @Hobbes mentioned the recent news about 2016 HO3 - a near-Earth asteroid that stays near Earth because its orbit around the sun is coupled to Earth. Thus the term quasi-satellite.
https://www.youtube.com/watch?v=SbbAnVU4rmY
In this NASA JPL video (above) the view is rotating around the sun following Earth. You can see the earth move slightly closer and farther from the sun since the Earth's orbit is not quite circular.
https://www.youtube.com/watch?v=2mVfE_qmQAg
This video (above) from http://arksky.org/calendar/alerts/714-what-is-it-the-strange-new-object-2016-ho3 shows a projection of 2016 HO3's motion against the stars as seen from Earth's location, but in a fixed direction. You can see the the sun and planets tend to follow the ecliptic, while 2016 HO3 does a figure-eight every year.
The NASA JPL news brief states:

The asteroid's orbit also undergoes a slow, back-and-forth twist over multiple decades. "The asteroid's loops around Earth drift a little ahead or behind from year to year, but when they drift too far forward or backward, Earth's gravity is just strong enough to reverse the drift and hold onto the asteroid so that it never wanders farther away than about 100 times the distance of the moon," said Chodas. "The same effect also prevents the asteroid from approaching much closer than about 38 times the distance of the moon. In effect, this small asteroid is caught in a little dance with Earth."
note: Paul Chodas is the manager of NASA's Center for Near-Earth Object (NEO) Studies

The item goes on to say:

Asteroid 2016 HO3 was first spotted on April 27, 2016, by the Pan-STARRS 1 asteroid survey telescope on Haleakala, Hawaii, operated by the University of Hawaii's Institute for Astronomy and funded by NASA's Planetary Defense Coordination Office. The size of this object has not yet been firmly established, but it is likely larger than 120 feet (40 meters) and smaller than 300 feet (100 meters).

First, I think it's quite cool that there even exists a Planetary Defense Office. It must be fun to go to work every day knowing that you are defending the planet!
I am interested in the story behind "...first spotted...".
The Pan STARRS telescope PS1 (http://pan-starrs.ifa.hawaii.edu/public/home.html) is managed by the PS1 Consortium. The original plan was for four telescopes, the status of PS2 is unclear. PS1 is a 1.8 meter telescope with a Gigapixel camera at the relatively fast f/4.4 focus, corrected for a very large 3° FOV, which means it's of the "primary with giant hole" variety (e.g. LSST). Typically it records an image every 30-60 seconds, and can record the entire night sky every three days.
I looked at the entry for 2016 HO3 (SPK-ID: 3752445) in the NASA JPL Small Body Data Browser and saw this:

The solution (shown on June 18) was calculated on June 11 2016 by an "O. Matic" :) using data going back to 2004.
As I said above, I am interested in the story behind "...first spotted...".
What were the sequence of events? Does "spotted" mean identified/flagged as an unknown object? Did it trigger a search of a database of earlier survey images from other telescopes? Is there a list available of the 80 total observations used? I'm curious if a twelve year old "first known image of 2016 HO3" exists.


texts: Relationship between temperature of nebula and size of star

texts: I was wondering, in the process of star formation, does the temperature of the nebula that produces a star play a role in the size of that star? I mean, it's only logical that the size would depend on the amount of matter around.But then, if the temperature is lower, that means that the net density is lower, so the star would 'absorb' less matter. Or is that not a factor, and the growing star would gobble up anything on its vicinity? I've encountered some contradicting sources and I'm trying to double check.


texts:
I am looking to create a function to correct for telluric interference and to do so I need to find data on Ozone interference.
It has been suggested to me that Astropy may have a function that does a similar thing and it would probably retrieve data of a high quality.
Does anyone know if such a thing exists and if so, how to determine where it is looking to get that data?

Thank you!
(I am new to python and astronomy as a whole so please take it easy on me if that's a bad question)


texts: Which eyepieces I can use it for best viewing experience using my existing telescope?

texts:
Telescope: Orion 09843 SpaceProbe 3,3 inch aperture(76mm), 700mm focal length, eyepieces - 25mm and 10mm focal lengths
Hi, I am very happy and excited to   see moon craters, Jupiter and Saturn. And planets like Mars, Venus, and Uranus are just looks
like a brighter star but not able to see its surface color. Similarly Jupiter looks like white small ball
instead of an orange surface. I am very much excited and  want to go bit closer. So please guide me list of eyepieces that I can use with this scope at least to see Jupiter  surface and big moon craters.


texts: Galactic orbits and distance between stars

texts:
In the context of whether or not nearby stars were created from the same nebula, this answer states:

imagine two stars with very similar orbits, one with a period of 200 million years and the other with a period of 210 million years. If they start off right next to each other, then after 2 billion years, the first star has made 10 complete orbits, while the second has made about 9.5 -- meaning it will now be on the other side of the galaxy from the first star.

How close would the two stars have originally been to have such different orbital periods?


texts: Why isn't the dark energy getting decreased?

texts: AS the articles on the web suggest, Dark energy is the reason behind the expansion of universe. If so, why isn't it getting used up in doing so. And consequently, the rate of expansion should reduce!


texts: Signal-to-noise in inverse angstrom for spectroscopy?

texts: In a paper I came across the description of the signal-to-noise ratio (SNR) for observations with a spectograph. This was reported as $10\:1/\mathring{A}$. I am rather new to spectroscopy, so could someone clarify these units for me? Why is it used and how should I read it?


texts: How can I map the zenith for an observer at a given time to equatorial coordinates (as would be measured in J2000)?

texts:
I'm writing a script to generate a star chart for a given observation point (latitude and longitude) at a given time.  I've found a database of stars with their right ascension and declination from J2000, and with that written a program that displays a star chart with the center being any right ascension and declination that I feed into it.  I've also written a program that will convert local time into local sidereal time. Now I just need to figure out how to determine the zenith for an observer in J2000 right ascension and declination given their longitude, latitude, and local sidereal time.

I've found a wikipedia page that gives formulas for converting between celestial coordinate systems, but the formula for converting equatorial and horizontal coordinates doesn't seem to depend on the sidereal time, which I imagine it should for my case.

I've tried finding a vector that represents the observer's zenith at the given time, then rotating it about the earth's rotation axis by the sidereal time, but that's giving incorrect results.


texts: What is the Northernmost Latitude of Saturn?

texts:
My wife and I were watching a TV show last night, which takes place in Salem, Massachusetts circa 1693.  One character went to a particular place in the woods at a particular time to be exactly under Saturn.  I told my wife that Saturn could never be exactly above such a Northern latitude.

What is the Northernmost Latitude that Saturn could possibly be above?  By "above", I mean if you draw a line through the center of the Earth and through Saturn, the point on the Earth's surface that line passes through.

Also, how often would it reach that point?  Would it be every few years, or every few thousand years?


texts: What is the significance of the discovery of a pulsar flipping between radio and x-ray emissions?

texts:
This morning on the radio I heard about the discovery of a pulsar that, due to its proximity to a companion star, was flipping between a state where it emitted radio waves and one where it emitted x-rays. The flipping between states was said to be due to material from the companion star falling onto the pulsar, causing it to rotate more quickly.

What is the significance of this discovery?

This newspaper article from the UK on the discovery contains more information, but doesn't go into detail about why this discovery is important.


texts:
I recently purchased a Celestron Astromaster EQ130 telescope and have absolutely no clue about telescopes but would love to learn.
I have set this up but cannot even get a decent view of the moon, any advice of help would be very much appreciated.
Thanks in advance.


texts: Is it true that we can estimate the mass of a star more accurately if it has a companion star?

texts: I remember I hear a theory that if a star has a companion star, we can know the mass of the star more easily, is that true? if it is true, while I understand that by measuring the period of the companion star, we can know the mass ratio of 2 stars,but how can the companion star also help to measure the mass of parent star?


texts: Could the known universe be inside a black hole and it's curvature really be the event horizon of the black hole?


texts: Searching dataset for Constellation detection

texts: I need to develop an Machine Learning (ML) model for predicting constellation seen in a picture taken from a mobile. This I need to develop from scratch. Firstly, I need help finding a database which can provide me images of stars with constellations labelled in it. Those can be from a ground based telescope or from a camera. This is going to be a multi-labelled problem. Even after spending hours of searching I can't find a good dataset. I want a dataset with several images (preferably a few 100), each labelled with the constellation seen in them. Then I am planning to refine them and use TensorFlow for prediction. I am a grade 12 student and still new to ML, so I really need your help.


texts: About the formation of ultra diffuse galaxies (UDGs), what are "failed L∗ galaxies"?

texts:
Recent studies show that ultra diffuse galaxies or UDGs were formed by tidally disturbed dwarf galaxies and failed L∗ galaxies.
So I've been searching for the failed L∗ galaxies but can't find what it is.
Are failed L∗ galaxies just the word that indicating Milky way like galaxies??


texts: Are there ways other than the collapse of a star which have been observed to form black holes?

texts: Every time I hear about a black hole, it's always in conjunction with the collapse of a star. Have any other processes been observed to create a black hole?


texts: What factors makes a star big in size(Physically)?Is the size of the nebula a relevant factor?

texts:
(Our sun compared to some of the known stars)

I know that the star is born in a nebula.Do only a extremely gigantic nebula give rise to large-radius star or is there any other factors related?


texts: Half of the Milky Way's baryonic mass is from other galaxies - when did this happen?

texts:
The recent Phys.org article Milky Way's origins are not what they seem explains the recent MNRAS article Anglés-Alcázar et al. 2017; The cosmic baryon cycle and galaxy mass assembly in the FIRE simulations DOI:(https://doi.org/10.1093/mnras/stx1517), also in ArXiv.

  Using supercomputer simulations, the research team found a major and unexpected new mode for how galaxies, including our own Milky Way, acquired their matter: intergalactic transfer. The simulations show that supernova explosions eject copious amounts of gas from galaxies, which causes atoms to be transported from one galaxy to another via powerful galactic winds. Intergalactic transfer is a newly identified phenomenon, which simulations indicate will be critical for understanding how galaxies evolve.

This is quite a sizable paper about quite a sizable simulation, and I'm having difficulty even getting started to read it.

Question I wanted to know, for a Milky Way-like galaxy, was most of the influx from other galaxies early on, or is this something that is predicted to happen somewhat steadily throughout the galaxy's lifetime?

Galaxy m12i in the simulation represents a Milky Way-mass galaxy, and so I show a screen shot of one of the figures for that galaxy below. Already I am stuck because I don't understand what $z$ represents. It seems to be related to time, rather than red-shift and $z=0$ has a special meaning, but I don't know if that's the present, or the beginning of the simulation.

below: Selected parts of Figure 3 from here. The selected column is data for galaxy m12i, which is representative of a Milky Way-mass galaxy in the simulation.

  Figure 3. Top panels: contribution of different processes to the evolution of galaxies m11 (left), m12i (middle), and m13 (right) from early times down to z = 0. Total stellar mass (top), ISM gas mass (middle), and gas accretion rate onto the galaxy (bottom) are indicated by the thick gray lines for each galaxy. Lines of different colors show the contributions from fresh accretion (purple), NEP wind recycling (blue), intergalactic transfer of gas (green), and galaxy mergers (ISM gas: orange; stars: red). All quantities represent average values over a timescale of ∼ 200 Myr. Bottom panels: fraction of stellar mass, ISM gas mass, and gas accretion rate contributed by each process as a function of redshift. Fractions are represented in terms of ranges, so that the contribution of each component at any redshift is given by the vertical extent of the corresponding color at that redshift.


texts: How can you determine the date using nothing but a star chart from the day and the time?

texts:
I'm doing a science olympiad event called Reach for the Stars, and one question that I've encountered provides a star chart and time of day, and you must figure out what month of the year it is from. The strategy that I developed includes figuring out which direction is west (usually a star chart from the night and the Sun sets in the west), then going in that direction estimating which zodiac constellation in Sun is in (and in turn approximately what month it is).
However, I realize that this method would not work if it was taken late at night, much after the Sun sets, because although I am fairly sure that each hour the sky rotates about 15 degrees, I can't estimate what constellation the Sun would be in.
Is there a more definite way to figure this out?
Edit: The time in the example chart is 8:30 PM in the time zone of the observer
Example star chart:


texts:
As I understand it, the distribution of matter is roughly homogeneous across space -- anywhere I stand in the universe, there will about the same amount of matter in every direction around me.

Is the distribution of matter across velocities roughly homogeneous?  If I flew away from the center of the Milky Way at .999c, would all directions look roughly the same, or would I notice a wind of galaxies and neutrinos traveling at a preferred velocity?

Tags I cannot add: cosmological principle, homogeneity, isotropy


texts: Can anyone explain this odd Galactic dust filament near the LMC?

texts: Perusing the Planck 353 GHz polarisation Aitoff projection I noticed an apparent magnetically driven dust filament rising abruptly from the Galactic ß-field in the Chameleon-Mensa S polar region. It streams for ~25° before dwindling in Eridanus. It appears to be associated with Galactic cirrus clumps 80 PC below the disc plane. The Planck 353 GHz image is here, and a quickie Chromoscope survey in optical, NIR, FIT mm/µm, and 21 cm is here. It is mentioned in only a few papers incl. Besla/Martinez-Delgado 2015, but there has been no specific study of its unusual morphology. It looks like a magnetically driven jet but originates from a region devoid of high-energy activity. If it is a Birkeland current it is a very odd one. I wonder if it is an old SN 1a remnant of the Hercules Loop and Aquila Rift type based on faint extinction filaments in Schlegel 1998 S Polar projection and 4 other Planck Aitoffs which seem to suggest traces of an old circular feature. Any suggestions?


texts: What is the official term for a relationship between a planet and its moons?

texts:
Sorry if this is a dumb question, I could not find anything naming this in my astronomy books, and Google searches only seemed to show me pages about astrology, [sarc] because naturally, human relationships are influenced by celestial bodies. [/sarc]

I'm thinking along the lines of a parent-child relationship or a symbiotic relationship, but I couldn't find anything that specifically names this kind of relationship.

Is there an official term for this relationship?


texts:
How could Hawking get the conclusion "[...] and black holes have positive energy. That’s why empty space is stable. Bodies such as stars or black holes cannot just appear out of nothing. But a whole universe can."? It would make sense to me, if a black hole has negative energy.

Quote from book The Grand Design, page 180:

  If the total energy of the universe must always remain zero, and it costs energy to create a body, how can a whole universe be created from nothing? That is why there must be a law like gravity. Because gravity is attractive, gravitational energy is negative: One has to do work to separate a gravitationally bound system, such as the earth and moon. This negative energy can balance the positive energy needed to create matter, but it’s not quite that simple. The negative gravitational energy of the earth, for example, is less than a billionth of the positive energy of the matter particles the earth is made of. A body such as a star will have more negative gravitational energy, and the smaller it is (the closer the different parts of it are to each other), the greater this negative gravitational energy will be. But before it can become greater than the positive energy of the matter, the star will collapse to a black hole, and black holes have positive energy. That’s why empty space is stable. Bodies such as stars or black holes cannot just appear out of nothing. But a whole universe can.


texts: Why does the moon 'peel' away from the horizon in this photograph?

texts:
The Himawari-8 geostationary weather satellite recently had a lunar intrusion into it's primary sensor (the Advance Himawari Imager, or AHI):

Zooming in on the moon:

I see a strange cupping (or 'peeling') of the moon away from the horizon:

My question is: what is the explanation for this 'peeling' aberration?

Note: if your answer involves the lunar terminator, please account for the following concerns:

At the time of day of the photograph, we see the Earth with ~5 degrees of longitude in night. The Earth presents approximately 17 degrees across from GEO, so the moon is $17^{\circ}/2=8.5^{\circ}$ degrees away from nadir, and so I expect the moon to have $5^{\circ}+8.5^{\circ}= 13.5^{\circ}$ of longitude in night. With barely half the moon peaking out above the Earth's horizon, the terminator is well behind the Earth.
The cupping aberration appears equal on both edges. The Sun is in the top right corner of the picture, and so I would expect a terminator related effect
to be more pronounced on the bottom of the moon than the top.


texts: Has the permittivity or the permeability of space been measured in a gravitational well?

texts:
Technically one might say yes since earth is gravitational well. However I am looking for more data points. For example,

in deep space where voyager might be

at the Lagrange points of massive bodies where gravitational fields are in equilibrium

at the bottom of a deep well like Jupiter

The intuition behind this question is the compactification of space that is caused by gravity. Certainly time slows in a gravitational well. And since c is constant, distances are relatively smaller. Please note I am not asking about em waves nor their velocity. Rather the distribution of field lines vs distance from a charge, a moving charge or a magnet.


texts: Could Planet Nine be much smaller than thought previously?

texts:
In 2016, Caltech astronomers began testing a Planet Nine hypothesis to explain the eccentric and inclined orbits of distant dwarf planets such as Sedna, 2015 TG387, Niku, and 2012 VP113.  The hypothesis stated that these objects are being shepherded by an undiscovered ice giant planet called "Planet 9" about 3.5 times larger and 10 times the mass of Earth.

Recently, an alternative hypothesis has surfaced suggesting that the Planet Nine object does not exist and that the orbits of said dwarf planets are being tugged by the collective mass of other, low-eccentricity objects. But this hypothesis also has it's downsides. The combined mass of the entire Kuiper belt of dwarf planets and asteroids is positively puny compared to the mass of Planet Nine, so it wouldn't be able to effect dwarf planets like Sedna.

But could it be both ways? The majority of these high-eccentricity plutoids are suspected to be in the inner Oort Cloud, a theorized ring of comets and possibly minor planets in the outskirts of the solar system. The combined mass of the inner Oort Cloud (or Hills Cloud) would be roughly 2.37 Earth masses if an estimate of 5 Earth masses is taken for the entire Oort Cloud. If the theorized Planet 9 was only about 7.63 times the mass of Earth, the gravitational pull of it and the Hills Cloud combined would be enough to change the orbits of Sedna-like objects.

This hypothesis more or less worked out in a Universe Sandbox 2 simulation I ran to test the theory, but would it actually be true, considering the inner Oort Cloud extends far beyond the orbit of Sedna?


texts:
The sun dominates in our solar system.
I wonder whether in every aspect the sun plays the most important role in our system.

For example, all kinds of metals are mainly located in the sun instead of the rest combined together in our system?

Is there any complementary relation between the sun and its children (all the planets, asteroids etc..)?
I mean: for one kind of metal, if the sun has more, her children should have less.

According to Abundance in the Sun of the elements, at least Li and Tb abundance of the sun are very low ($\approx1e-8$). Furthermore, these abundances are from the solar atmosphere which is near the surface of the sun.


texts: Where does energy at the beginning of a star's lifecycle (before any nuclear reactions) come from?

texts:
David Christian's Maps of Time has this to say about the period during which the first stars started to form from the diffuse clouds of hydrogen and helium that then made up the universe:

  Under the pull of their own gravity, the clouds of hydrogen and helium began to collapse in on themselves...As gravity packed each cloud into ever smaller spaces, pressure built up in the center. Increasing pressure means increasing temperatures, and so, as they shrank, each gas cloud began to heat up (~Kindle location 1469).

The part I'm working on understanding is "Increasing pressure means increasing temperatures." Does the pressure itself cause the molecules to heat up? Or is it the kinetic energy that the particles pick up as gravity pulls them towards the center of the cloud? Or is the phenomenon here something other than increased energy per unit mass?

Of course, once the nuclear chain reaction gets started in the star it's not hard to figure out where the energy comes from. I'm talking about before that.


texts: Why nuclear fusion is a controlled process in stars?

texts: Nuclear fusion is the main way for stars to generate energy. But we know that nuclear fission and fusion are not controlled, then it causes chain reaction and sudden explosion. But why in the case of stars it does not happen? I mean, that nuclear fusion happen in stars but it take few billion years to complete the amount of hydrogen. Is there any controlling mechanism? If no, what happens here?